Annular fluids and method of emplacing the same

ABSTRACT

An annular fluid or packer fluid, and methods of making the same, that includes a water-miscible solvent, a viscosifying additive, a crosslinking agent, a crosslinking inhibitor having the facility to inhibit crosslinking between the viscosifying additive and the crosslinking agent, and an initiating agent having the facility to overcome an action of the crosslinking inhibitor and to initiate crosslinking between the viscosifying additive and the crosslinking agent, is shown and described. The fluid has a thermal conductivity of no more than about 0.25 btu/(hr·ft·° F.) and a potential to substantially increase its viscosity upon sitting for a selected period of time.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to viscosifiable, low thermalconductivity annular fluids and methods of viscosifying, emplacing, andremoving the fluids.

2. Background Art

Annular fluids or packer fluids are liquids which are pumped into anannular opening between a casing and a wellbore wall or betweenadjacent, concentric strings of pipe extending into a wellbore. Thesefluids are especially necessary in oil or gas well constructionoperations conducted in low temperature venues of the world, forexample, those areas having permafrost. Permafrost is a thick layer offrozen surface ground which may be several hundred feet thick andpresents a great obstacle to the removal of relatively warm fluidsthrough a well pipe. Particularly, warm fluid in the well pipe causesthawing of the permafrost in the vicinity of the well resulting insubsidence which can impose compressive and/or tension loads high enoughto rupture or collapse the well casing and hence allow the escape ofwell fluids. In addition, the warm gas or oil coming to the surface inthe well pipe becomes cooled by giving up its heat to the permafrost.Further, gas hydrate crystals may form, which can freeze together andblock the well pipe. Generally, except for a tiny contribution fromradiation, annular heat loss is due to convection and to conduction.

Heavy oil production is another operation which often can benefit fromthe use of an insulating annular fluid. In heavy oil production, ahigh-pressure steam or hot water is injected into the well and the oilreservoir to heat the fluids in the reservoir, causing a thermalexpansion of the crude oil, an increase in reservoir pressure and adecrease of the oil's viscosity. In this process, damage to the wellcasing may occur when heat is transferred through the annulus betweenthe well tubing and the casing. The resulting thermal expansion of thecasing can break the bond between the casing and the surrounding cement,causing leakage. Accordingly, an insulating medium such as a packerfluid may be used to insulate or to help insulate the well tubing. Thepacker fluid also reduces heat loss and saves on the energy requirementsin steam flooding.

In addition to steam injection processes and operations which requireproduction through a permafrost layer, subsea fields—especially, subseafields in deep water, 1,500 to more than 6,000 feet deep—requirespecially designed systems which typically require a packer fluid. Forexample, a subsea oil reservoir temperature may be between about 120° F.and 250° F., while the temperature of the water through which the oilmust be conveyed is often as low as 32° F. to 50° F. Conveying the hightemperature oil through such a low temperature environment can result inoil temperature reduction and consequently the separation of the oilsinto various hydrocarbon fractions and the deposition of paraffins,waxes, asphaltenes, and gas hydrates. The agglomeration of these oilconstituents can cause blocking or restriction of the wellbore,resulting in significant reduction or even catastrophic failure of theproduction operation.

To meet the above-discussed insulating demands, a variety of packerfluids have been developed. For example, U.S. Pat. No. 3,613,792describes an early method of insulating wellbores. In the U.S. Pat. No.3,613,792, simple fluids and solids are used as the insulating medium.U.S. Pat. No. 4,258,791 improves on these insulating materials bydisclosing an oleaginous liquid such as topped crude oils, gas oils,kerosene, diesel fluids, heavy alkylates, fractions of heavy alkylatesand the like in combination with an aqueous phase, lime, and a polymericmaterial. U.S. Pat. No. 4,528,104 teaches a packer fluid comprised of anoleaginous liquid such as diesel oil, kerosene, fuel oil, lubricatingoil fractions, heavy naphtha and the like in combination with anorganophillic clay gellant and a clay dispersant such as a polar organiccompound and a polyfunctional amino silane. U.S. Pat. No. 4,877,542teaches a thermal insulator fluid consisting of a heavy mineral oil asthe major liquid portion, a light oil as a minor liquid portion, asmectite-type clay, calcium oxide and hydrated amorphous sodiumsilicate. U.S. Pat. No. 5,290,768 teaches a thixotropic compositioncontaining ethylene glycol and welan gum. The above-discussed patentsare herein incorporated by reference.

Although many of the above-described packer fluids function adequately,they fail to meet industrial and governmental concerns for theenvironment. Particularly, many of the constituents of theabove-described packer fluids are unacceptable from an environmentalstandpoint and are often prohibited for use by government regulation.For example, the mineral oils and heavy crude oils required by severalof the above-discussed patents are not permitted for use in areas suchas the Gulf of Mexico.

Further attempts at providing insulating annular fluids based on fluidshaving a more acceptable HSE profile are discussed in G.B. Patent2,367,315 by Vollmer (hereinafter referred to as “Vollmer '315”).Nothing is taught about insulating annular fluids in U.S. Pat. No.5,304,620 by Holtmyer, et al. (hereinafter referred to as “Holtmyer'620”), U.S. Pat. No. 5,439,057 by Weaver, et al. (hereinafter referredto as “Weaver '057”), and U.S. Pat. No. 5,996,694 by Dewprashad, et al.(hereinafter referred to as “Dewprashad '694”). However, Holtmyer '620,Weaver '057, and Dewprashad '694 discuss the viscosification of brinesusing crosslinked hydroxyethylcellulose derivatives. These patents arehereby incorporated by reference.

What is needed, however, are insulating packer fluids that are very lowin thermal conductivity while simultaneously meeting all of the otherconstraints imposed upon the packer fluids and are easier to pump, yetbecome more viscous than conventional fluids when the insulating packerfluids are resident in situ within the annular space or one of theannular spaces in an oil or gas well.

SUMMARY OF INVENTION

In one aspect, the present invention relates to a packer fluid thatincludes a water-miscible solvent, a viscosifying additive, acrosslinking agent having the facility to crosslink the viscosifyingadditive, a crosslinking inhibitor adapted to inhibit crosslinkingbetween the viscosifying additive and the crosslinking agent, and aninitiating agent having the facility to overcome an action of thecrosslinking inhibitor and to initiate crosslinking between theviscosifying additive and the crosslinking agent. In a particularaspect, the packer fluid has a thermal conductivity of no more thanabout 0.25 btu/(hr·ft·° F.), and a potential to substantially increaseits viscosity upon sitting for a selected period of time.

In another aspect, the present invention relates to a method forpreparing a packer fluid that includes mixing a water-miscible solventand a viscosifying additive to produce a first fluid, acidifying thefirst fluid with an acid to produce a second fluid, adding acrosslinking agent to the second fluid to produce a third fluid, andadding an initiating agent to the third fluid to permit crosslinkingbetween the viscosifying additive and the crosslinking agent.

In another aspect, the present invention relates to a method foremplacing a packer fluid into an annulus that includes preparing apacker fluid, pumping the packer fluid into the annulus before theselected period of time, and allowing the packer fluid to sit in theannulus such that the viscosity of the packer fluid is substantiallyincreased.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

DETAILED DESCRIPTION

In one aspect, the present invention relates to insulating packerfluids, and methods of emplacing and subsequently removing such fluids.Packer fluids according to the present invention have very low thermalconductivities, while simultaneously meeting other constraints (e.g.,regulatory or environmental constraints) imposed upon the packer fluids.These fluids are facile to pump, yet are capable of becoming veryviscous after they are resident in situ within the annular space or oneof the annular spaces in an oil or gas well.

As noted above, a majority of annular heat loss is due to convection andconduction. Heat loss due to convection can be arrested or substantiallydiminished by increased viscosities of the fluids. On the other hand,heat loss due to thermal conductivity needs to be controlled by properselection of fluids.

Prior art packer fluids often are oil-based (hydrocarbon-based) becauseoil-based fluids typically have very low thermal conductivities. Forexample, thermal conductivities as low as 0.07 btu/(hr·ft·° F.) can beobtained with gelled diesel or other hydrocarbon-based insulatingannular fluid. As noted above, these fluids typically have adverseenvironmental effects and are not desirable. Therefore, water-based andwater-miscible fluids are, in many cases, preferred in spite of the factthat water-based fluids typically have much higher thermalconductivities because water has a thermal conductivity of 0.351btu/(hr·ft·° F.). The thermal conductivities, λ, of a wide variety ofaqueous fluids, including water, can be approximated to within about ±8%as follows:λ[btu/(hr·ft·° F.)]=−0.16262·ρ+0.51929 Or λ[Watts/(cm·° C.)]=−281.45×10⁻⁵·ρ+898.76×10⁻⁵where ρ=the fluid density, in gm/cm³

While most aqueous solutions have thermal conductivities as predicted bythis equation, some exceptions do exist. For example, concentratedsulfuric acid (90%) has a thermal conductivity that is 8.8% below thepredicted value, and a 30% dilute sulfuric acid has a thermalconductivity that is 16.5% below the predicted value. Similarly, thermalconductivities of concentrated sucrose solutions fall substantiallybelow the predicted values (up to about 29% below the predicted values).The same phenomenon is also observed with non-aqueous water-misciblesolutions such as ethylene glycol, propylene glycol, and solutions ofvarious salts such as calcium bromide in ethylene glycol or propyleneglycol. These solutions can have thermal conductivities that are as muchas 60% lower than the predicted values.

For example, ethylene glycol has a ρ of 1.1108 gm/cm³ and a λ of 0.149btu/(hr·ft·° F.), substantially lower than the predicted value ofλ=0.339 btu/(hr·ft·° F.) based on its density. Similarly, a solution of139 ppb calcium bromide in ethylene glycol has a ρ of 1.388 gm/cm³ and aλ of 0.133 btu/(hr·ft·° F.), substantially lower than the predictedvalue of λ=0.294 btu/(hr·ft·° F.) based on its density. For comparison,similar calcium bromide brines (i.e., in water instead of ethyleneglycol) have λ values of 0.34 and 0.30 btu/(hr·ft·° F.), respectively,when the densities are 1.1108 gm/cm³ and 1.388 gm/cm³. Because theethylene glycol-based fluids have low thermal conductivity and arerelatively environmentally friendly, the applicability of these andrelated fluids as insulating annular fluids was investigated by thepresent inventors.

Embodiments of the present invention relate to low thermal conductivityannular fluids based on water-miscible fluids (e.g., various glycols),which have been formulated to have the desired Theological properties.Specifically, these annular fluids are formulated to have certainviscosities for facile pumping of these fluids into an annular space.However, they will develop substantially increased viscosities in situafter they have been emplaced in the annular space. In other words,these fluids have delayed viscosifying properties. Various formulationsof fluids within the scope of the present invention are provided belowas examples. However, the present invention is not limited to thedescribed embodiments, but is bounded by the claims that follow.

EXAMPLE 1

A fluid having a density of 9.86 ppg and a λ of about 0.14 btu/(hr·ft·°F.) was formulated from the following components:

1. Ethylene glycol 322.87 gm  78.0% by wt.  2. ECF 680 16.8 gm 4.1% bywt. 3. Concentrated HCl 2.38 gm 0.6% by wt. 4. CaCl₂ (dry) 70.0 gm 16.9%by wt.  5. MgO  2.0 gm 0.5% by wt.

ECF 680 is a slurry of a doubly derivatized hydroxyethyl cellulose(DDHEC) in an inert, water-miscible carrier fluid. ECF 680 is availablecommercially from M-I L.L.C. (Houston, Tex.). DDHEC may be synthesizedby grafting monomers of vinyl phosphonic acid (VPA) onto cellulosepolymers according to methods disclosed in U.S. Pat. No. 5,304,620(Holtmyer '620).

The components were added in the order listed in the table. The firsttwo components, ethylene glycol and ECF 680, were mixed and stirredtogether for about 1 hour to thoroughly disperse the polymer in theethylene glycol. Then, 2.38 grams of concentrated (˜38.5 wt %)hydrochloric acid were added and stirring was continued for about 30minutes. The mixture was set aside and its viscosity was observed overthe course of 4 hours. At first, the viscosity was approximately that ofethylene glycol, but after about 2 hours, the viscosity increasedsubstantially, to about 300 cP when measured at 200 rpm on a Fann 35Aviscometer.

Next, dry calcium chloride was added, liberating heat as the soliddissolved in the mixture. The calcium chloride was added in a controlledfashion, to avoid thermally degrading the DDHEC. After the mixture hadcooled to room temperature, the viscosity was measured again to be about300 cP when measured at 200 rpm on a Fann 35A viscometer. Finally, themagnesium oxide, in the form of a moderately reactive solid product(e.g., moderately fine powder), was added as a slurry in a few grams ofethylene glycol.

It has been discovered that magnesium oxide neutralizes the acid andraises the pH to the 8-9 range, whereupon divalent or multi-valentcations (e.g., Ca²⁺) already present in the mixture crosslink DDHEC.Alternatively, other polymers similar to DDHEC could be used (e.g.,similarly modified cellulose, guar, or hydroxypropyl guar), and themulti-valent or divalent cations may be added separately. Also, it willbe obvious to one skilled in the art that other methods may be used toeffect the same result—for example, applying a mildly complexedcrosslinkant which becomes slowly un-complexed in order to effect theinitial crosslinking, and subsequently applying a stronger complexingadditive to effect the breaking (un-crosslinking).

The crosslinking does not occur immediately, but instead occurs over thecourse of several hours, leading to doubling of the apparent viscosityof the mixture during these several hours and gradually increasing toabout 50 percent of its ultimate value upon sitting overnight at roomtemperature. Only after sitting at room temperature for several days didthe mixture achieve its ultimate viscosity, that of a semi-rigid gel.The delayed viscosifying property of this embodiment permits facileemplacement of this fluid into an annular space, while the ultimateviscosity of this fluid is desirable for arresting or substantiallyreducing heat loss due to convection.

EXAMPLE 2

Because the embodiment of Example 1 has desirable properties for use asan annular fluid, the process of initiating polymer crosslinking byaddition of MgO is investigated in more detail. In the second example, afluid having a density of 9.86 ppg and a λ of about 0.14 btu/(hr·ft·°F.) was formulated similarly to Example 1, except that the magnesiumoxide was not added as a slurry in ethylene glycol but as a dry powder.The magnesium oxide was found to be able to disperse thoroughlythroughout the mixture without causing any locally high pH pocket thatmight lead to premature crosslinking. As in Example 1, the crosslinkingoccurred over the course of several hours, leading to an increase in theviscosity.

EXAMPLE 3

Next, the fineness of the MgO powder is investigated. In the thirdembodiment, a fluid having a density of 9.86 ppg and a λ of about 0.14btu/(hr·ft·° F.) was formulated similarly to Example 1, except that themagnesium oxide was a very fine powder in a highly reactive form, i.e.,having small particle size, high surface area, and ready accessibilityfor reaction. One example of such a fine powder MgO is availablecommercially from M-I L.L.C under the trade name of Di-Balance™. Thefine powder MgO was added as a slurry in ethylene glycol. The magnesiumoxide was found to be able to disperse thoroughly throughout the mixturewithout causing any locally high pH pocket that might lead to prematurecrosslinking. As in Example 1, the crosslinking occurred over the courseof several hours, leading to an increase in the viscosity.

The above examples show that crosslinking may be initiated with theaddition of MgO in various forms.

EXAMPLE 4

In the above examples, the divalent cation (Ca²⁺) is prevented fromcrosslinking DDHEC by the addition of acid (HCl). Subsequent addition ofMgO counters the effect of HCl and allows the crosslinking to takeplace. Surprisingly, the order of the addition of the divalent cationand the acid has a dramatic effect on the ability of the resultant fluidto viscosify, as evidenced by the following example. In this example, afluid having a density of 9.86 ppg and a λ of about 0.14 btu/(hr·ft·°F.) was formulated from the same components as in Example 1, but in thefollowing order:

1. Ethylene glycol 322.87 gm  2. ECF 680 16.8 gm 3. CaCl₂ (dry) 70.0 gm4. Concentrated HCl 2.38 gm 5. MgO  2.0 gm

The components were added in the order listed in the table. The firsttwo components (ethylene glycol and ECF 680) were mixed and stirredtogether for about 1 hour to thoroughly disperse the polymer into theethylene glycol. Then the dry calcium chloride was added, liberatingheat as the solid dissolved in the mixture. The calcium chloride wasadded in a controlled manner to prevent the temperature from rising highenough to thermally degrade the DDHEC.

After the mixture had cooled to room temperature, it was observed to beonly slightly more viscous than ethylene glycol. Next, 2.38 grams ofconcentrated (˜38.5 wt %) hydrochloric acid were added and stirring wascontinued for about 30 minutes. The mixture was then set aside and itsviscosity was observed over 4 hours. After 4 hours, the viscosity hadnot increased substantially, indicating that the DDHEC had beeninhibited from properly hydrating in the ethylene glycol fluid due tothe presence of calcium chloride prior to the addition of the acid.

Finally, magnesium oxide, in the form of a highly reactive solidproduct, for example, that is available commercially from M-I L.L.C.,was added. However, unlike in Example 1, the viscosity of the mixtureonly increased very slowly and only to a relatively small extent. Onlyafter sitting at room temperature for several days did the mixtureachieve its ultimate viscosity.

Even after reaching its ultimate viscosity, the fluid could still bereadily poured from one container to the next, whereas the fluid ofExample 1 at a comparable point in time had become a semi-rigid gelwhich could not be poured. This embodiment demonstrates that the orderof addition of the components is important, and it is preferred thatacid be added before the addition of the divalent cation.

EXAMPLE 5

Annular fluids of the above examples thicken by crosslinking actionbetween DDHEC and Ca²⁺. Thus, it is expected that the counter anion ofthe calcium salt should not have much effect. This is shown to be thecase in the following example. In this example, a fluid having a densityof 10.8 ppg and a λ of about 0.20 btu/(hr·ft·° F.) was formulated asfollows:

1. 120.92 gms. of calcium bromide brine (about 52 wt % CaBr₂) (26.7% bywt.)

2. 29.08 gms. of water (6.4% by wt.)

3. 10.0 gms. of ethylene glycol (2% by wt.)

4. 16.8 gms. of ECF 680 (3.7% by wt.)

Note that calcium bromide substitutes for calcium chloride in thisformulation. The reagents were added in the above order. The mixture wasthen stirred for about 30 minutes. To this mixture, the following wasadded:

5. 2.38 gms. (2.0 ml) of concentrated HCl (0.5% by wt.)

The mixture was stirred until it thickened significantly, then it wasput aside for ˜1 hour. While the above mixture was sitting, in a secondjar the following was mixed:

6. 235.77 gms. of ethylene glycol (52.2% by wt.)

7. 36.74 gms. of calcium bromide (dry) (8.1% by wt.)

A detectable increase in heat resulted upon the addition of the calciumbromide to the ethylene glycol. The mixture in the second jar wasstirred while the mixture first heated, then cooled (step 7). When theproduct of step 7 had cooled to room temperature, 8.0 grams of thesolution was removed. To the removed 8.0 grams of solution, thefollowing was added (step 8):

8. 2.0 gms. of MgO. (0.4% by wt.)

While the 2 grams of MgO was being mixed with the 8.0 grams of solution(step 8), the rest of the product from step 7 was rapidly mixed into thethick slurry produced in step 5 (step 9).

When these two fluids were thoroughly mixed, at least 95% of the slurryfrom step 8 was mixed into the product of step 9 (step 10).

The product of step 10 was then set aside so that the crosslinkingreaction could proceed. Crosslinking did not occur immediately, but overthe course of several hours, leading to the doubling of the apparentviscosity of the mixture during these several hours. The apparentviscosity of the mixture gradually increased to about 50 percent of itsultimate value upon sitting overnight at room temperature. Only aftersitting at room temperature for several days did the mixture achieve itsultimate viscosity, that of a semi-rigid gel.

In this example, calcium bromide is added in a form with some water,which allows the acidified polymer to hydrate and yield fully even inthe presence of a substantial concentration of salt. Results from thisexample show that calcium bromide may be used instead of calciumchloride without appreciable changes in the fluid viscosity.

EXAMPLE 6

In the above example, calcium bromide was added partly as a brine andpartly as a dry powder. The following example shows that relativeproportions of these two forms of calcium bromide do not havesignificant effects on the resultant fluid properties. In this example,a fluid having a density of 10.8 ppg and a λ of bout 0.20 btu/(hr·ft·°F.) was formulated as follows:

1. 90.69 gms. of calcium bromide brine (about 52 wt % CaBr₂) (20% bywt.)

2. 21.81 gms. of water (4.8% by wt.)

3. 100.0 gms. of ethylene glycol (22% by wt.)

4. 16.8 gms. of ECF 680 (3.7% by wt.)

The reagents were added in the above order. The mixture was then stirredfor about 30 minutes. To this mixture, the following was added (step 5):

5. 2.38 gms. (2.0 ml) of concentrated HCl (0.5% by wt.)

The mixture was stirred until it thickened significantly, then was putaside for ˜1 hour. While the above mixture was sitting, in a second jarthe following was mixed:

6. 168.56 gms. of ethylene glycol (39.2% by wt.)

7. 51.48 gms. of calcium bromide (dry) (i. e., additional CaBr₂ to thatof step 1). (11.3% by wt.)

A detectable increase in heat resulted upon the addition of the calciumbromide to the ethylene glycol. The mixture in the second jar wasstirred while the mixture first heated, then cooled (step 7). When theproduct of step 7 had cooled to room temperature, 8.0 grams of thesolution was removed. To the removed 8.0 grams of solution, thefollowing was added (step 8):

8. 2.0 gms. of MgO. (0.4% by wt.)

While the magnesium oxide was becoming wetted with the 8.0 grams ofsolution, the remaining product of step 7 was mixed into the thickslurry produced in step 5 (step 9). When these two fluids werethoroughly mixed, at least 95% of the slurry from step 8 was mixed intothe rapidly mixed product of step 9 (step 10).

The product of step 10 was then set aside so that the crosslinkingreaction could proceed. Crosslinking did not occur immediately, but overthe course of several hours, leading to the doubling of the apparentviscosity of the mixture during these several hours. The apparentlyviscosity of the mixture gradually increased to about 50 percent of itsultimate value upon sitting overnight at room temperature. Only aftersitting at room temperature for several days did the mixture achieve itsultimate viscosity, that of a semi-rigid gel. This example shows thatrelative proportions of calcium bromide used have no significant effecton the ultimate viscosity of the fluid.

EXAMPLE 7

In embodiments of the invention, ultimate viscosities of the annularfluids result from crosslinking DDHEC by the divalent ions. Thus, it isexpected that the amounts of DDHEC in these fluids should havesignificant effects on the final viscosities of the fluids. Thefollowing two examples illustrate the effects of the DDHECconcentrations on the viscosities of the resultant fluids. A fluidhaving a similar density and thermal conductivity as those in examples 5and 6, i.e., density of 10.8 ppg and a λ of about 0.20 btu/(hr·ft·° F.),was formulated as follows:

1. 90.69 gms. of calcium bromide brine (20% by wt.)

2. 21.81 gms. of water (4.8% by wt.)

3. 100.0 gms. of ethylene glycol (22% by wt.)

4. 12.6 gms. of ECF 680 (i. e., only 75% of that used in Example 6)(2.8% by wt.)

The reagents were added in the above order. The mixture was then stirredfor about 30 minutes. To this mixture, the following was added (step 5):

5. 2.38 gms. (2.0 ml) of concentrated HCl (0.5% by wt.)

The mixture was stirred until it thickened significantly, then was putaside for ˜1 hour. While the above mixture was sitting, in a second jarthe following was mixed:

6. 172.70 gms. of ethylene glycol (38% by wt.)

7. 51.53 gms. of calcium bromide (dry) (11.3% by wt.)

A detectable increase in heat resulted upon the addition of the calciumbromide to the ethylene glycol. The mixture in the second jar wasstirred while the mixture first heated, then cooled (step 7). When theproduct of step 7 had cooled to room temperature, 8.0 grams of thesolution was removed. To the removed 8.0 grams of solution, thefollowing was added (step 8):

8. 2.0 gms. of MgO. (0.4% by wt.)

While the magnesium oxide was becoming wetted with the 8.0 grams ofsolution, the remaining product of step 7 was mixed into the thickslurry produced in step 5 (step 9). When these two fluids werethoroughly mixed, at least 95% of the slurry from step 8 was mixed intothe product of step 9 (step 10). The product of step 10 was then setaside so that the crosslinking reaction could proceed.

Crosslinking did not occur immediately, but over the course of severalhours. The crosslinking did not lead to the doubling of the apparentviscosity of the mixture during these several hours, but to asubstantially increased viscosity. Gradually the viscosity of themixture increased to about 50 percent of its ultimate value upon sittingovernight at room temperature, but to a somewhat lesser value than thatin Example 6. Only after sitting at room temperature for several daysdid the mixture achieve its ultimate viscosity, that of a weakly rigidgel. This example shows that the ultimate viscosity of the fluid islower when DDHEC is reduced by 25%.

EXAMPLE 8

This example investigates the effect of further reduction in DDHEC onthe ultimate viscosity of the fluid. Accordingly, a fluid having adensity of 10.8 ppg and a λ of about 0.20 btu/(hr·ft·° F.) wasformulated as follows:

1. 90.69 gms. of calcium bromide brine (20% by wt.)

2. 21.81 gms. of water (4.8% by wt.)

3. 100.0 gms. of ethylene glycol (22% by wt.)

4. 8.4 gms. of ECF 680 (i. e., only 50% of that used in Example 6) (1.9%by wt.)

The reagents were added in the above order. The mixture was then stirredfor about 30 minutes. To this mixture, the following was added (step 5):

5. 2.38 gms. (2.0 ml) of concentrated HCl (0.5% by wt.)

The mixture was stirred until it thickened significantly, then was putaside for ˜1 hour. While the above mixture was sitting, in a second jarthe following was mixed:

6. 176.85 gms. of ethylene glycol (39% by wt.)

7. 51.58 gms. of calcium bromide (dry) (11.3% by wt.)

A detectable increase in heat resulted upon the addition of the calciumbromide to the ethylene glycol. The mixture in the second jar wasstirred while the mixture first heated, then cooled (step 7). When theproduct of step 7 had cooled to room temperature, 8.0 grams of thesolution was removed. To the removed 8.0 grams of solution, thefollowing was added (step 8):

8. 2.0 gms. of MgO. (0.4% by wt.)

While the magnesium oxide was becoming wetted with the 8.0 grams ofsolution, the remaining product of step 7 was mixed into the thickslurry produced in step 5.

When these two fluids were thoroughly mixed, at least 95% of the slurryfrom step 8 was mixed into the product of step 9 (step 10). The productof step 10 was then set aside so that the crosslinking reaction couldproceed.

Crosslinking did not occur immediately, but over the course of severalhours. The crosslinking did not lead not to the doubling of the apparentviscosity of the mixture during these several hours, but to asubstantially increased viscosity. Gradually the viscosity of themixture increased to about 50 percent of its ultimate value upon sittingovernight at room temperature, but to a lesser value than that inExample 6. Only after sitting at room temperature for several days didthe mixture achieve its ultimate viscosity, that of a weak but “lipping”gel. This example and Example 7 clearly show that ultimate viscositiesof these fluids are dependent on the concentrations of DDHEC.

EXAMPLE 9

Next, the effects of different salts are examined. A fluid having adensity of 10.8 ppg and a λ of about 0.16 btu/(hr·ft·° F.) wasformulated from the following components:

1. Ethylene glycol 335.55 gm  74.0% by wt.  2. ECF 680 16.8 gm 3.7% bywt. 3. Concentrated HCl 2.38 gm 0.5% by wt. 4. NaBr (dry) 96.89 gm 21.3% by wt.  5. MgO  2.0 gm 0.4% by wt.

This embodiment has a similar composition as that of Example 1, exceptthat sodium bromide is used instead of calcium bromide. The componentswere added in the order listed above. The first two components weremixed and stirred together for about 1 hour to thoroughly disperse thepolymer into the ethylene glycol. Then 2.38 grams of concentrated (˜38.5wt %) hydrochloric acid were added, and stirring was continued for about30 minutes. The mixture was set aside and its viscosity was observedover the ensuing 4 hours. At first the viscosity was approximately thatof ethylene glycol, but after about 2 hours the viscosity increasedsubstantially to about 300 cP when measured at 200 rpm on a Fann 35Aviscometer.

Next, dry sodium bromide was added (no significant liberation of heatwas observed). Finally, the magnesium oxide, for example, in the form ofa highly reactive solid product available commercially from M-I L.L.C.,was added. As in Example 1, over the course of several hours theapparent viscosity of the mixture doubled and gradually increased toabout 50 percent of its ultimate value upon sitting overnight at roomtemperature. Only after sitting at room temperature for several days didthe mixture achieve its ultimate viscosity, which is substantially thesame as that of Example 1. This example shows that substituting sodiumions for the divalent calcium ions did not result in any appreciablechange in the ultimate viscosity of the fluid. While the exact mechanismis not clear, it is possible that the added MgO provides sufficientdivalent cations for the crosslinking.

EXAMPLE 10

As noted above, the order of the addition of calcium bromide andhydrochloric acid has a significant impact on the ultimate viscosity ofthe fluid (see Example 4). The effect of reversing the addition ofsodium bromide and hydrochloric acid is investigated in the followingexample. A fluid having a density of 10.8 ppg and a λ of about 0.16btu/(hr·ft·° F.) was formulated from the following components:

1. Ethylene glycol 335.55 gm  74.0% by wt.  2. ECF 680 16.8 gm 3.7% bywt. 3. NaBr (dry) 96.89 gm  21.3% by wt.  4. Concentrated HCl 2.38 gm0.5% by wt. 5. MgO  2.0 gm 0.4% by wt.

This composition comprises identical components in the same amounts asthose in Example 9. The components were added in the order listed above.The first two components were mixed and stirred together for about 1hour to thoroughly disperse the polymer into the ethylene glycol. Thendry sodium bromide was added (no significant liberation of heat wasobserved). Next, 2.38 grams of concentrated (˜38.5 wt %) hydrochloricacid were added, and stirring was continued for about 30 minutes.

The mixture was set aside and its viscosity was observed over theensuing 4 hours. At first the viscosity was approximately that ofethylene glycol, but after about 2 hours the viscosity had increasedsubstantially to about 80 cP when measured at 200 rpm on a Fann 35Aviscometer. Finally, the magnesium oxide, for example, in the form of ahighly reactive solid product available commercially from M-I L.L.C.,was added. Unlike Example 9, the viscosity of the mixture only increasedvery slowly and to a lesser extent. Only after sitting at roomtemperature for several days did the mixture achieve its ultimateviscosity, that of a relatively weak gel, whereas the fluid of Example 9at a comparable point in time had become a semi-rigid gel which couldnot be poured.

Like Example 4, this example shows that the order of addition of thecomponents is important, and, for best results, the salt (e.g., sodiumbromide or calcium bromide) addition should be delayed until after theacid has been added. However, unlike in the case of calcium chlorideaddition (e.g., example 4), the salt and sodium bromide did notcompletely inhibit the hydration, yielding, and subsequent crosslinkingof the polymer.

EXAMPLE 11

The above examples show that embodiments of the invention graduallythicken at room temperature over time. When these fluids are actuallyemplaced in an annular space, they might experience significantly highertemperatures. Therefore, the ability of these fluids to thicken at ahigher temperature is investigated. A fluid having a density of 9.86 ppgand a λ of about 0.14 btu/(hr·ft·° F.) was formulated from the followingcomponents:

1. Ethylene glycol 322.87 gm  78.0% by wt.  2. ECF 680 16.80 gm  4.1% bywt. 3. Concentrated HCl 2.38 gm 0.5% by wt. 4. CaCl₂ (dry) 70.0 gm 16.9%by wt.  5. MgO 2.00 gm 0.5% by wt.

This composition is identical to that of Example 1. The components werealso added in the same order as in Example 1, i.e., as listed above. Thecomponents were mixed as described in Example 1. However, after themagnesium oxide was added, the fluid was not allowed to sit overnight atroom temperature to crosslink. Instead, the mixture was placed in a 180°F. oven for one hour, taken out and cooled to room temperature, and itsviscosity was assessed. At this point, the viscosity had visiblyincreased, indicating the onset of crosslinking. However, at this point,the fluid was still readily pourable from one container to another.

The mixture was placed in a 180° F. oven for another hour, taken out andcooled to room temperature, and its viscosity was assessed. At thispoint, the gel had become very weakly “lipping” and could only be pouredvery slowly from one container to another. Finally, the mixture wasplaced in a 180° F. oven for two additional hours, taken out and cooledto room temperature, and its viscosity was assessed. At this point, thegel had become strongly “lipping.” Only after sitting at roomtemperature overnight did the mixture achieve its ultimate viscosity,that of a semi-rigid gel. This example shows that higher temperatureswould not impede the thickening process. On the contrary, it may speedup the thickening process. Accordingly, the temperature factor may needto be taken into account in emplacing the fluids of the presentinvention.

The above examples show that embodiments of the invention can providepacker fluids or annular fluids which have low thermal conductivities,can be pumped easily, and will become substantially viscosified uponemplacement, whereupon they will become substantially incapable ofconducting heat through convection, and consequently will have low totalheat-loss.

Embodiments of the invention use water miscible glycols to formulatefluids with low thermal conductivities. Although the examples useethylene glycols, one of ordinary skill in the art would appreciate thatother glycols (e.g., diethylene glycol, triethylene glycol,tetraethylene glycol, propylene glycol, dipropylene glycol, tripropyleneglycol, tetrapropylene glycol, and the like), alcohol-glycol ethers(e.g., ethylene glycol monobutyl ether, methyl diethylene glycol, ethyltriethylene glycol, propyl tetraethylene glycol, ethyl propylene glycol,methyl dipropylene glycol, propyl tripropylene glycol, and the like), oralcohols may be used without departing from the scope of the invention.In addition, a mixture of such water miscible solvents may be used. Theexact concentrations of the water miscible solvents may depend on thedesired thermal conductivities and the type of water miscible solvents.In the above examples, ethylene glycol is used at a concentration of54%-78% by weight in the final fluids. These fluids have thermalconductivities of 0.2 btu/(hr·ft·° F.) or less. In examples 5-8, up toabout 10% by weight of water is included, while ethylene glycol ispresent at about 54%-61% by weight. These fluids, nevertheless, haverelatively low thermal conductivities (about 0.2 btu/(hr·ft·° F.)).

In the above examples, doubly derivatized hydroxyethyl cellulose (DDHEC)is used to thicken the fluids. The derivatization is by reacting HECwith vinyl phosphonic acid. One of ordinary skill in the art wouldappreciate other similar polymers may be used, such as derivatizedhydroxypropyl cellulose or derivatized cellulose. These polymers thickenupon crosslinking with divalent or polyvalent ions. Other polymers thatthicken by different mechanisms may also be used. For example, guars orcelluloses, with or without derivatization, may be crosslinked withboron, titanium, or zirconium. DDHEC may be conveniently used in aslurry form, such as that sold under the trade name of ECF 680™ by M-I,L.L.C. (Houston, Tex.).

The amounts of DDHEC used in the above examples range from about 1.9% toabout 3.7% by weight of ECF 680™. The ultimate viscosities of the fluidsdepend upon the amounts of DDHEC used. The above examples clearly showthat high viscosities (e.g., that of a semi-solid) can be achieved at aDDHEC concentration of about 3.7% by weight of ECF 680™. Even at lowerconcentrations of DDHEC, appreciable thickening of the fluids wasachieved.

The divalent or polyvalent ions that crosslink DDHEC or the like maycome from salts (e.g., CaBr₂) that are present in the fluids before aLewis base or a Bronsted-Lowry base (e.g., MgO) is added to initiate thecrosslinking, or from the Lewis base or a Bronsted-Lowry base (e.g.,MgO) itself. While MgO is used in the above examples, one of ordinaryskill in the art would appreciate that other Lewis base or aBronsted-Lowry base, such as BaO, CaO, or the like, may be used withoutdeparting from the scope of the invention. In addition, alkaline metalor alkaline-earth metal hydroxides (e.g., NaOH, KOH, etc.) or similarbases may also be used to raise the pH and to initiate the crosslinkingif the fluids already include divalent or polyvalent ions.

Although unexpected, embodiments of the invention clearly show that theorder of addition of various components may have a significant impact onthe ultimate viscosities of the resultant fluids. It is preferred thatthe salts (e.g., CaBr₂ or NaBr) be added after the acid has been added.Otherwise, the fluids may not achieve the desired viscosities. If thesalts (or brines) were to be added before the addition of acid, the saltsolutions or brines should be adjusted to an acidic pH range (i.e.,pH<7) before mixing with the viscosifying polymer (e.g., DDHEC).

While the above examples illustrate the procedures for preparingembodiments of the invention on a laboratory scale, these procedures aretypically modified in field application. The following illustrates oneexample of how to prepare an annular fluid on location according toembodiments of the present invention. Prior to emplacement, an annularfluid using, for example, the DDHEC polymer as the viscosifier additive,may be prepared as follows:

1. Before use, all DDHEC polymer slurry pails are stirred to thoroughlyre-disperse any settled polymer into the slurry. The slurry preferablyis stirred until it is smooth and its color is uniform before adding toa completion fluid or other fluids.

2. The required volumes of glycol, brine, fluid, and/or fresh water areloaded into the blender. Stock fluid is preferred because ironcontamination can impede polymer hydration. Any dry salts needed forpreparing the required fluid are added at this time and allowed to fullydissolve. The fluid is warmed to between 60° F. and 80° F. Fluidtemperatures below 60° F. will slow hydration of the polymer and candelay crosslinking.

3. The pH of the completion fluid is checked and, if necessary, adjustedwith lime or hydrochloric acid to between 5-7.

4. The DDHEC polymer slurry is added into the blender over the top. Atypical dosage is one ˜5.4 gallon pail (˜50 lb) per 3 bbl fluid.

5. The circulation and agitation is continued for 10 minutes tothoroughly disperse the slurry before adding hydrochloric acid. Then thepH of the mixture is adjusted to the 1-2 range with 26% hydrochloricacid. About ½ gallon of acid per 3 bbl fluid is added at a time andmixed for at least 5 minutes before checking pH or proceeding withfurther additions. The use of excessive acid can lessen the overalllongevity of the polymer.

6. If foaming or aeration of gel is excessive, a small amount ofdefoaming additive may be added to eliminate the foam.

7. When the mixture develops some viscosity (about 20-30 cP),circulation through the centrifugal pump may be discontinued, but slowagitation with the paddle is continued. Extended circulation through thecentrifugal pump can break down polymer backbone due to excessive shearstress—and/or due to the combined stress of very low pH and highshear—and lessen the overall longevity of the polymer. The mixture isallowed to hydrate until fully yielded (hydrated), as evidenced by nofurther visual change of the gel viscosity in the blender. The finalviscosity is typically about 160 to 240 cP, depending on fluid andtemperature. In some cases, hydration should be complete within 20 to 90minutes, especially with fluids having substantial amounts of freewater; for other fluids, especially those having a higher glycolcontent, hydration may take longer.

8. While allowing the polymer in the mixture to hydrate, the magnesiumoxide is prepared for addition to the mixture. The recommended magnesiumoxide loading is about 1 lb/bbl of gelled fluid. For non-calcium-basedfluids or application temperatures above 200° F., 1.25 lb/bbl ofmagnesium oxide is recommended. Using a small container such as a5-gallon bucket, the required amount of the magnesium oxide is mixedinto 11.6 ppg CaCl₂ brine at 10 lbs/3 gal.

9. The magnesium oxide/fluid slurry is preferably used within 2 hoursafter mixing. Otherwise, it will harden if stored for later use.

10. Once the DDHEC polymer is fully hydrated, the gel is circulatedthrough the eductor port on the top of the blender. While circulating,the magnesium oxide slurry is slowly added into the blender through theeductor port. The jetting action of the circulating gel helps to evenlydisperse the magnesium oxide slurry. If sufficient crosslinking has notbegun to occur within 15 minutes, slurry an additional ¼ lb./per bbl. ofthe magnesium oxide for addition to the mixture through the eductorport.

At this time, when some viscosity has developed, but well before asubstantial fraction of the fluid has had an opportunity to crosslink,the mixture is ready for emplacement. Once significant crosslinking hasoccurred, the product will become much more difficult to pump.Therefore, to avoid any danger of having the mixture “lock-up” in thewell and in the piping leading to the annulus into which it is to beemplaced, the crosslinking process should not be initiated (by theaddition of the magnesium oxide) until timing is appropriate for promptplacement of the mixture.

Crosslink time is influenced by several factors including:

-   -   a. Fluid type        -   High glycol content fluids crosslink more slowly.        -   High alcohol, alcohol-ether, or polyol content fluids            crosslink more slowly.        -   Calcium-based brines crosslink more quickly, decreasing            crosslink time.        -   Increased density decreases crosslink time for calcium            brines.    -   b. Temperature    -    Warmer fluids will crosslink more quickly.    -   c. The magnesium oxide concentration        -   Increasing the magnesium oxide concentration decreases            crosslink time.

Once the fluid is emplaced in the desired location in the annulus of thewell, any excess fluid in connecting piping, valves, manifolds, etc.,should be displaced before the fluid has had time enough to thoroughlycrosslink. After the fluid has been emplaced in the annulus and givensufficient time to crosslink, the fluid will remain essentially free ofconvective currents that are responsible for convective heat-lossthrough the annulus. The only remaining heat-loss avenues are radiativeheat-loss, which is ordinarily negligible in magnitude, and conductiveheat-loss, which is minimized through the selection of alow-thermal-conductivity base fluid for the mixture.

After the fluid has been emplaced in the annulus, there may becircumstances that require intervention—a work-over is needed, a pipecollapses, develops a leak, or parts, etc. If this should happen, amethod for external breaker addition is provided through which theemplaced crosslinked fluid may be broken and removed so that theintervention can take place without the interfering presence of remnantcrosslinked fluid. This method entails washing 10-15% hydrochloric acidthrough the crosslinked fluid. A suitable amount of hydrochloric acidfor the washing may depend on several factors; an exemplary amount isabout 25 gallons of hydrochloric acid per foot of zone. The applicationof the dilute hydrochloric acid lowers the pH of the mixture into a pHrange where the crosslinking is no longer effective, and whereupon thefluid breaks. Thereafter, the remaining fluid can be circulated out ofthe well bore.

Hydrochloric acid can be spotted upon or atop the crosslinked fluidinstead, but this method requires a much longer break time. Several daysmay be required to fully break the mixture if diffusion is relied uponto distribute the hydrochloric acid to break through the crosslinkedfluid.

The example discussed above describes a crosslinking and un-crosslinking(breaker) mechanism which is controlled by a pH swing. Other methods ofeffecting the same result—for example, applying a mildly complexedcrosslinkant which becomes slowly un-complexed in order to effect theinitial crosslinking, and subsequently applying a stronger complexingadditive to effect the breaking (un-crosslinking)—will be obvious to oneskilled in the art. Furthermore, other means of breaking the viscosity,such as, for example, the application of an oxidizing breaker likelithium hypochlorite or sodium chlorite or the application of acombination of an oxidizing breaker and acid, will be obvious to oneskilled in the art.

Thus, the present invention advantageously discloses novel compositionsfor use as insulating packer fluids and methods for emplacing the same.Insulating packer fluids are designed to reduce heat loss due toconduction and convection when emplaced in the annular space in an oilor gas well or to assist in reducing heat loss due to conduction andconvection when emplaced in one of the annular spaces in an oil or gaswell.

In contrast, conventional insulating packer fluids reduce conductiveheat loss primarily by being formulated from base fluids that haveinherently low coefficients of thermal conductivity, and they reduceconvective heat loss primarily by being formulated with viscosifyingadditives that are so viscous from the very beginning, once the fluid isfully formulated, and throughout the useful life of the fluid, thatconvection currents are arrested or substantially diminished. However,conventional insulating packer fluids often cannot achieve the lowestinherent heat loss because of other constraints on the composition orproperties of the fluid, for example, that it be non-hydrocarbon-basedfor environmental reasons or for compatibility with elastomer elementspresent in the wellbore, or that it be sufficiently low in viscositythat the horsepower requirements for pumping at a reasonable rate do notexceed the capacities of available pumping means.

Furthermore, conventional insulating packer fluids often cannot compriseviscosifying additives that are so viscous that convection currents arearrested or substantially diminished because such viscosified fluidsbecome too viscous to pump with available power supplies, pumping means,flow lines, connectors, and other associated hardware.

Embodiments of the present invention advantageously provide insulatingpacker fluids that are very low in thermal conductivity whilesimultaneously meeting all of the other constraints imposed upon thepacker fluid. Preferred embodiments of the present invention are basedon compositions that are 25 to 100% ethylene glycol (or other suitablechemicals having the requisite properties described above).

In addition, embodiments of the present invention are easier to pump yetbecome more viscous than conventional fluids when the insulating packerfluids are resident in situ within the annular space or one of theannular spaces in an oil or gas well. This is accomplished byincorporating a crosslinkable viscosifying additive into the novelcompositions taught herein. Other compositions in accordance with thepresent invention comprise blends of conventional insulating packerfluids with those that include crosslinkable viscosifying additives.

While reference has been made to a limited number of crosslinkableviscosifying agents for use in a low thermal conductivity medium, it isexpressly within the scope of the present invention that a variety ofpolymers and crosslinking agents may be used. Typical brine-based wellfluid viscosifying additives include natural and synthetic polymers andoligomers. The viscosifying additives suitable for embodiments of thepresent invention include poly(ethylene glycol) (PEG), poly(diallylamine), poly(acrylamide), poly(aminomethylpropyl-sulfonate [AMPS]),poly(acrylonitrile), poly(vinyl acetate), poly(vinyl alcohol),poly(vinyl amine), poly(vinyl sulfonate), poly(styryl sulfonate),poly(acrylate), poly(methyl acrylate), poly(methacrylate), poly(methylmethacrylate), poly(vinylpyrrolidone), poly(vinyl lactam), and co-,ter-, quater-, and quinque-polymers of the following co-monomers:ethylene, butadiene, isoprene, styrene, divinylbenzene, divinyl amine,1,4-pentadiene-3-one (divinyl ketone), 1,6-heptadiene-4-one (diallylketone), diallyl amine, ethylene glycol, acrylamide, AMPS,acrylonitrile, vinyl acetate, vinyl alcohol, vinyl amine, vinylsulfonate, styryl sulfonate, acrylate, methyl acrylate, methacrylate,methyl methacrylate, vinylpyrrolidone, vinyl lactam, vinyl phosphonate,bis-(β-chloroethyl vinyl phosphonate) {also known as bis(2-chloroethyl)vinylphosphonate}, bis (hydrocarbyl) vinylphosphonate,1,1-dichlorovinylethyl phosphate, 1,1-dichlorovinylethyl phosphate,triethanolamino-bis-chlorophosphoric acid, hydrophilic monomers of theformula I

wherein R¹ is hydrogen, methyl or ethyl, R² is the group —COOR⁴, thesulfonyl group, the phosphonyl group, the phosphonyl group esterified by(C₁-C₄)-alkanol or a group of the formula

wherein R³ is hydrogen, methyl, ethyl or the carboxyl group, R⁴ ishydrogen, amino or hydroxy-(C₁-C₄)-alkyl and R⁵ is the sulfonyl group,the phosphonyl group or the carboxyl group, or grafting(co)polymerization of one or more hydrophilic monomers of the formula Ionto a grafting base, using a free radical initiator which forms threeor more free radical sites per molecule, or monomers of the formula II:X₂O₃PCHYCZ₂PO₂XHwhere X is H, alkali metal or ammonia, Y and Z are each H, PO₃X₂, SO₃Xor CO₂X (e.g., vinyl phosphonic acid or vinylidene diphosphonic acid) orgroups which react with hypophosphorous acid in the presence of freeradicals providing compounds X₂O₃PCHYCZ₂PO₂XH which react with monomerssuch as vinyl sulfonate, vinylphosphonate, vinylidene diphosphonate andacrylic acid.

The viscosifying additives suitable for embodiments of the presentinvention also include “natural” polymers onto which have been graftedone or more co-, ter-, and quater-monomers of the following: ethylene,butadiene, isoprene, styrene, divinylbenzene, divinyl amine,1,4-pentadiene-3-one (divinyl ketone), 1,6-heptadiene-4-one (diallylketone), diallyl amine, ethylene glycol, acrylamide, AMPS,acrylonitrile, vinyl acetate, vinyl alcohol, vinyl amine, vinylsulfonate, styryl sulfonate, acrylate, methyl acrylate, methacrylate,methyl methacrylate, vinylpyrrolidone, vinyl lactams, vinyl phosphonate,bis-(β-chloroethyl vinyl phosphonate) {also known as bis(2-chloroethyl)vinylphosphonate}, bis (hydrocarbyl) vinylphosphonate,1,1-dichlorovinylethyl phosphate, 1,1-dichlorovinylethyl phosphate,triethanolamino-bis-chlorophosphoric acid, hydrophilic monomers of theformula I

wherein R¹ is hydrogen, methyl or ethyl, R² is the group —COOR⁴, thesulfonyl group, the phosphonyl group, the phosphonyl group esterified by(C₁-C₄)-alkanol or a group of the formula

wherein R³ is hydrogen, methyl, ethyl or the carboxyl group, R⁴ ishydrogen, amino or hydroxy-(C₁-C₄)-alkyl and R⁵ is the sulfonyl group,the phosphonyl group or the carboxyl group, or grafting(co)polymerization of one or more hydrophilic monomers of the formula Ionto a grafting base, using a free radical initiator which forms threeor more free radical sites per molecule, or monomers of the formula II:X₂O₃PCHYCZ₂PO₂XHwhere X is H, alkali metal or ammonia, Y and Z are each H, PO₃X₂, SO₃Xor CO₂X (e.g., vinyl phosphonic acid or vinylidene diphosphonic acid) orgroups which react with hypophosphorous acid in the presence of freeradicals providing compounds X₂O₃PCHYCZ₂PO₂XH which react with monomerssuch as vinyl sulfonate, vinylphosphonate, vinylidene diphosphonate andacrylic acid.

Suitable crosslinking agents in accordance with the present inventioninclude (1) “crosslinking agents active upon vicinal diol groups”, i.e.,a borate, titanate, or zirconate crosslinkants as taught in U.S. Pat.No. 5,062,969, (2) divalent, trivalent, or tetravalent cations such as,for example, Fe²⁺, Cd²⁺, Co²⁺, Ca²⁺, Cu²⁺, UO₂ ²⁺, PbO²⁺, Al³⁺, Fe³⁺,Cr³⁺, Ce³⁺, Ti⁴⁺, Zr⁴⁺, Sn⁴⁺, and the like, (3) complexes of or othermoieties containing the crosslinkants listed above in the first twocategories, such as, for example, the tetrammine complex of the Cu²⁺cation, the carbonate anion complexes of the UO₂ ²⁺ cation, UO₂(CO₃)₂ ²⁻and UO₂(CO₃)₃ ⁴⁻, or the triethanolamine complex of the Ti⁴⁺ cation, (4)so-called “organic crosslinkants” such as, for example, formaldehyde,and glutaraldehyde, and (5) mixtures of the crosslinkants listed abovein the first four categories and/or reaction products therefrom.

Furthermore, it will be clear to one of ordinary skill in the art thatother brine solutions, such as ZnCl₂, CaCl₂, CaBr₂, ZnBr₂, NaHCO₂,KHCO₂, CsHCO₂, NaCl, KCl, NH₄Cl, MgCl₂, seawater, NaBr, KBr, CsBr, andcombinations thereof may be used in connection with the presentinvention.

Furthermore, while the foregoing embodiments reference a limited numberof water miscible low thermal conductivity compounds as the base fluid,one of ordinary skill in the art will recognize that chemical compoundshaving the same general characteristics also will function in ananalogous fashion. For example, it is expressly within the scope of thepresent invention that other compounds containing primary, secondary, ortertiary alcohols may be used, such as, diethylene glycol, triethyleneglycol, and other glycol derivatives like diethylene glycol methylether,diethylene glycol ethylether, triethylene glycol methylether, andtriethylene glycol ethylether, glycerol and glycerol derivatives likeglycerol formal, glycerol 1,3 diglycerolate, glyceroethoxylate, 1,6,hexandiol, and 1,2 cyclohexandiol.

In contrast to the prior art, embodiments of the present inventiondisclose making an annular fluid as thick as possible prior tocrosslinking so that the fluid is readily pumpable. Further, embodimentsof the present invention disclose subsequently adding a crosslinkingactivator during the process of pumping the fluid so that the fluid willnot gain viscosity due to crosslinking while the fluid is being pumped,but will gain viscosity to a very substantial extent later when thefluid is resident in situ within the annular space or one of the annularspaces in an oil or gas well. Therefore, embodiments of the presentinvention advantageously provide an annular fluid that controls annularheat loss due to convection and to conduction.

Further, while reference has been made to the addition of crosslinking“promoters,” it is expressly within the scope of the present inventionto use fluids having a thermal conductivity of no more than about 0.25btu/(hr·ft·° F.), which include a water-miscible solvent and aviscosifying additive, wherein the packer fluid has an inherentcapability to substantially increase its viscosity upon sitting for aselected period of time. In certain embodiments, therefore, low thermalconductivity fluids may be used that viscosify simply after beingemplaced in a well or through other triggering mechanisms known to thosein the art.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A method for preparing a packer fluid, comprising: mixing awater-miscible solvent and a viscosifying additive to produce a firstfluid; acidifying the first fluid with an acid to produce a secondfluid; adding a crosslinking agent to the second fluid to produce athird fluid; and adding an initiating agent to the third fluid to permitcrosslinking between the viscosifying additive and the crosslinkingagent.
 2. A method for preparing a packer fluid, comprising: mixing awater-miscible solvent and a crosslinking agent to produce a firstfluid; adjusting a pH of the first fluid to produce a second fluidhaving a pH of about 5 to about 7; adding a viscosifying additive to thesecond fluid to produce a third fluid; adjusting a pH of the third fluidusing an acid to a range of about 1 to 2 to produce a fourth fluid; andadding an initiating agent to the fourth fluid to permit crosslinkingbetween the viscosifying additive and the crosslinking agent.
 3. Amethod for emplacing a packer fluid into an annulus, comprising:preparing the packer fluid comprising: a water-miscible solvent; aviscosifying additive; a crosslinking agent having the facility tocrosslink the viscosifying additive; a crosslinking inhibitor having thefacility to inhibit crosslinking between the viscosifying additive andthe crosslinking agent; and an initiating agent having the facility toovercome an action of the crosslinking inhibitor and to initiatecrosslinking between the viscosifying additive and the crosslinkingagent, wherein the packer fluid has a thermal conductivity of no morethan about 0.25 btu/(hr·ft·° F.), and a potential to substantiallyincrease its viscosity upon sitting for a selected period of time;pumping the packer fluid into the annulus before the selected period oftime; and allowing the packer fluid to sit in the annulus such that aviscosity of the packer fluid is substantially increased.